Re: [PATCH 5/8] x86/domctl: Stop using XLAT_cpu_user_regs()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.03.2025 17:01, Andrew Cooper wrote:
> On 17/03/2025 11:38 am, Jan Beulich wrote:
>> On 11.03.2025 22:10, Andrew Cooper wrote:
>>> In order to support FRED, we're going to have to remove the {ds..gs} fields
>>> from struct cpu_user_regs, meaning that it is going to have to become a
>>> different type to the structure embedded in vcpu_guest_context_u.
>>>
>>> In both arch_{get,set}_info_guest(), expand the 
>>> memcpy()/XLAT_cpu_user_regs()
>>> to copy the fields individually.  This will allow us to eventually make them
>>> different types.
>>>
>>> No practical change.  The compat cases are identical, while the non-compat
>>> cases no longer copy _pad fields.
>> That's fine for "set", but potentially not for "get": Someone simply doing
>> memcmp() on two pieces of output might then break.
> 
> It's not a fastpath, and I'm not looking to not break things, but I was
> expecting it to be safe.
> 
> The pad fields for cs (inc saved_upcall_mask) and ss get lost on the
> first exit-from-guest, and the pad fields for the data segment get lost
> on the first schedule.

Are they? If these fields on the stack are only every written with zero
(which aiui they are), all vCPU-s would properly observe zero in the
padding fields.

> So while there is a change here, I don't think it's anything that
> current code could plausibly be relying on.
> 
> Furthermore, when we get rid of the vm86 fields, we don't even store the
> pad fields anywhere in Xen, so they're going, one way or another, by the
> end of the series.
> 
> Finally, disaggregation or not, this is an unstable interface so we do
> have some wiggle room.
> 
> I guess I should discuss this more in the commit message?

Yes, if you continue to be convinced that dropping of their copying is
fine, the justification of that would be very desirable to have in the
description.

>>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>> ---
>>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>>> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>>
>>> Should we really be copying error_code/entry_vector?  They're already listed
>>> as explicitly private fields, and I don't think anything good can come of
>>> providing/consuming them.
>> I don't see a reason why we'd need to copy them in arch_set_info_guest();
>> arch_set_info_hvm_guest() doesn't copy them either. For
>> arch_get_info_guest() it's less clear - toolstack components may have
>> grown a dependency on them (e.g. introspection?), so I'd err on the side
>> of retaining prior behavior. (Of course there's then the corner case of
>> someone calling "get" right after "set", expecting the two fields to come
>> back unchanged.)
> 
> Introspection doesn't use this interface.  Regs are sent in the ring,
> and don't contain these fields either.
> 
> Also, for HVM guests, we set the vmexit rsp to &error_code so we only
> push the GPRs, without the IRET frame above it.
> 
> These fields, (inc saved_upcall_mask) have different behaviours under
> FRED.  I don't think we can get away without them changing, and for
> these at least, they were clearly marked as internal.

And you're reasonably convinced that in a tool like xenctx it couldn't
make sense to dump such simply for informational purposes?

Jan